Question

A format for textbook page layouts is to be chosen so that each printed page has a 4-cm margin at the top and at the bottom and a 2-cm margin on the left and right sides. The rectangular region of printed matter is to have an area of 800 sq. cm. Find the dimensions for the page that has a minimum area.

let the width of the printed section by x
let its height be y
then xy=800
y=800/x

let A be the area of the whole page
A=(x+4)(y+8)
=(x+4)(800/x + 8)

expand, differentiate, set that equal to zero and solve

I got x=20

for y i got 72
thanx, im not good at setting up equations